The secondary battery in which an alkali metal such as lithium is used as an active material of a negative electrode has generally various advantages. For example, it not only ensures high energy and high electromotive force, but also has wide operating temperature range due to the use of a nonaqueous electrolyte and is excellent in shelf life, miniaturized and lightweight.
Therefore, the practical use of the above lithium secondary battery with nonaqueous electrolytes is anticipated as a power source for use in a portable electronic appliance and also as a high-performance battery for use in an electric vehicle and electricity storage.
However, all the developed prototype batteries have not fully realized the above properties anticipated from the lithium secondary battery, and thus have been incomplete from the viewpoint of cycle life, charge and discharge capacities and energy density.
A major cause thereof resided in a negative electrode used in the secondary battery.
For example, a lithium secondary battery having a negative electrode composed of metal lithium incorporated therein has disadvantageously short cycle life and poor safety because lithium precipitated on the surface of the negative electrode during charging formed acicular dendrite is apt to cause short-circuit between the negative and positive electrodes.
Lithium has extremely high reactivity, thereby causing the electrolyte to suffer from decomposition reaction in the vicinity of the surface of the negative electrode. Thus, there was the danger that the above decomposition reaction would modify the surface of the negative electrode to thereby lower the cell capacity during repeated uses of the secondary battery.
Various studies have been made on the material of the negative electrode with a view toward obviating the above problems of the lithium secondary battery.
For example, the use of alloys containing lithium, such as lithium/aluminum and Wood's alloy, as the material of the negative electrode of the lithium secondary battery, has been studied. However, this negative electrode composed of such a lithium alloy had a problem of crystal structure change attributed to the difference in operating temperature and charge and discharge conditions.
Further, the use of graphite and other carbon materials as the material of the negative electrode of the lithium secondary battery, has been studied. For example, an attempt has been made to insert lithium ions formed during charging in spacing between graphite layers of the carbon material (intercalation) to thereby produce a compound known as "intercalation compound" for the purpose of preventing the formation of dendrite.
As such carbon materials, carbon fibers, especially carbon fibers derived from mesophase pitch, have been extensively studied and results superior to those of the conventional carbon materials have been reported. In this connection, reference is made to Japanese Patent Laid-Open Publication Nos. 4(1992)-61747, 4(1992)-184862 and 4(1992)-196055.
However, the carbon materials are various in the size and configuration of crystallites, the content of impurities, etc., depending on the type of the starting material and the manufacturing conditions. The process for producing the carbon material optimum for the formation of the electrode of the lithium secondary battery and the characteristics of the above carbon material, have not yet been specified, and the current situation is that no carbon material for use as a negative electrode which can satisfy all the demands on prolongation of the cycle life, increase of the charge and discharge capacities, etc. has been developed.
Various studies on the increase of the capacity of the secondary battery have been reported. However, there are few or no reports on the development directed to another key point, i.e., the cell volume or the energy efficiency per weight.
The development of a suitable casing and the improvement of the positive electrode and the electrolyte are inevitable for the decrease of the weight of the secondary battery. These are not sufficient, and the increase of the bulk density of the negative electrode is also an inevitable important matter.
The inventors have made extensive and intensive studies on the carbon material for use in the negative electrode which can prolong the cycle life of the secondary battery and can increase the charge and discharge capacities and the energy efficiency thereof. As a result, they have found that the configuration of the carbon material used in the negative electrode has striking impacts on not only the improvement of the cycle characteristics and charge and discharge capacities of the secondary battery having the above negative electrode incorporated therein but also the miniaturization of the secondary battery. The present invention has been completed on the basis of the above finding.